The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods for discharging a capacitor with automatic detection. Merely by way of example, the invention has been applied to a power converter. But it would be recognized that the invention has a much broader range of applicability.
Power converters have been widely used for consumer electronics such as portable devices. The power converters can convert electric power from one form to another. As an example, the electric power is transformed from alternate current (AC) to direct current (DC), from DC to AC, from AC to AC, or from DC to DC. Additionally, the power converters can also convert the electric power from one voltage level to another voltage level. Specifically, the power converters include linear converters and switch-mode converters. The switch-mode converters often use pulse-width-modulated (PWM) or pulse-frequency-modulated (PFM) mechanisms. The switch-mode converters usually are more efficient than the linear converters.
FIG. 1 is a simplified conventional diagram showing a switch-mode power conversion system with an AC input resistor and an AC input capacitor. The switch-mode power conversion system 100 includes an alternating current (AC) input resistor 120 (e.g., an X-resistor), an AC input capacitor 110 (e.g., an X-capacitor), input terminals 122 and 124, a switch-mode controller 130, a primary winding 172, a secondary winding 174, an auxiliary winding 176, and a switch 180.
As shown in FIG. 1, the switch-mode power conversion system 100 includes the AC input capacitor 110 that is connected to the input terminals 122 and 124. The AC input capacitor 110 often is used to address the issue of electromagnetic interference (EMI). Additionally, to comply with some safety requirements, the voltage across the AC input capacitor 110 needs to drop below a predetermined threshold (e.g., direct current 60V) in a short time period (e.g., within 1 second) after the input terminals 122 and 124 are disconnected from an AC power supply. To facilitate the discharge of the AC input capacitor 110, the power conversion system 100 also includes the AC input resistor 120 that is connected to the AC input capacitor 110 in parallel.
But the AC input resistor 120 also consumes power when the power supply for AC input voltage is not disconnected and thus causes the standby power consumption of the power conversion system 100 to rise. For example, if the AC input resistor 120 has a resistance of 3 MΩ, then, with a 264-volt AC input voltage, the power consumption of the resistor 120 is about 23 mW.
As shown in FIG. 1, after the input terminals 122 and 124 are disconnected from the AC power supply, the charges accumulated on the AC input capacitor 110 are released through the AC input resistor 120. For example, the voltage across the AC input capacitor 110 decreases with time as follows.
                                          V            xc                    ⁡                      (            t            )                          =                              V            0                    ×                      e                          -                              (                                  t                                                            R                      x                                        ⁢                                          C                      x                                                                      )                                                                        (                  Equation          ⁢                                          ⁢          1                )            where Vxc is the voltage across the AC input capacitor 110, and V0 is the voltage value of Vxc at the time when the input terminals 122 and 124 are disconnected from the AC power supply. Rx and Cx are the resistance value and the capacitance value of the AC input resistor 120 and the AC input capacitor 110 respectively.
As an example, in order for Vxc to drop approximately by a factor of e within one second, the resistance of the AC input resistor 120 satisfies the following equation.
                              R          x                ≈                  1                      C            x                                              (                  Equation          ⁢                                          ⁢          2                )            
Usually, the magnitude of Cx depends on the power of the switch-mode power conversion system 100 and the solution to electromagnetic interference. If the capacitance of the AC input capacitor 110 is increased, the resistance of the AC input resistor 120 would become smaller according to Equation 2 and the AC input resistor 120 may consume more power.
Hence it is highly desirable to improve the techniques of discharging the AC input capacitor in order to reduce the power loss.